All right? Tuning at Fs is part of the "classic" bass reflex that Thiele put up as the most desirable alignment! The configuration is: Qts = .383. Volume of Box = Vas, and tuning frequency = Fs, and F3 will equal Fs.
It your Qts is higher than .4, than the box should be larger than Vas and the tuning should be below Fs. These are optimal numbers-you can vary.
While alignments with Vb, (volume of box) less than Vas and tuning frequency abve Fs tend to be sightly preferred, (somewhat less distortion), it is perfectly okay to tune below Fs if you don't push it too far. Qts = 0.53 or so and Vb being 1.5 Vas is perfectly reasonable.
What is your speaker's Qts?
It your Qts is higher than .4, than the box should be larger than Vas and the tuning should be below Fs. These are optimal numbers-you can vary.
While alignments with Vb, (volume of box) less than Vas and tuning frequency abve Fs tend to be sightly preferred, (somewhat less distortion), it is perfectly okay to tune below Fs if you don't push it too far. Qts = 0.53 or so and Vb being 1.5 Vas is perfectly reasonable.
What is your speaker's Qts?
0.43. You've answered my question anyway.
However look at this...http://www.penhometheatre.com.au/castbooksketc.htm
Its using a 0.43 qts driver.
Its tuned above Fs. All the common bass alignments say a 0.43 Q driver should be tuned below FS.
why?
There are many different tuning alignments for ported systems. As a matter of fact the number of available alignments vary continuously from an over-damped B2 alignment to a C6 maximum-ripple with 2 pole active bass extension and beyond. A B4 ported alignment is where f3 = fb and Vas = Vb. The alignment you’re asking about will fall more in the QB3 to B2 range. Both are over damped alignments. A further analysis would require more specifics about the driver.
Tuning is always a trade-off of specification and design. It depends on the driver design and what you want from that driver. Bass extension, box size, and efficiency will always be in conflict with each other and you’ll find yourself trading one for the other.
Tuning is always a trade-off of specification and design. It depends on the driver design and what you want from that driver. Bass extension, box size, and efficiency will always be in conflict with each other and you’ll find yourself trading one for the other.
Tech:
This is an example similar to one given by David Weems, who has been explaining how the Thiele-Small numbers work for decades through his book, "Designing, Building and Testing Loudspeakers".
Suppose you have a 10" driver with the following specs:
Fs = 30 Hz
Vas = 2 cu ft
Qts = .38, (or .4, for simplicity)
The "optimal" box will be a 2 cu ft box tuned to 30 Hz. The response will be flat with the F3, (-3 dB point), being 30 Hz.
Suppose we want to use this same driver in a smaller box. Suppose we want to use it in a 1 cu ft box.
Remember, a box half the size will give an F3 one half octave above any given box. So our F3 will be raised half an octave, from 30 Hz to 42 Hz. (To raise a frequency half an octave, you multiply it by 1.4-not 1.5. To raise it a full octave, you multiply it by 2).
Our box will then be:
Vb=1 cu ft
Fb, (frequency you tune the box) = something slightly below 42 Hz
F3 = 42 Hz.
However you will have a +2.3 dB hump in the octave above 42 Hz, and your transient response will be a little worse than the "optimal" box. But-it still can be a good, listenable box. +2.3 dB is not that bad. There is a formula for the correct box tuning, but if you make it 39 or 40 Hz-a little below your F3 of 42 Hz-you should be very, very close.
Another way of looking at that 1 cu ft box is to say that the best Qts for that box is about .32, and the hump is caused by having a Qts too high-.43 instead of .32. But again, the box will be listenable, except for perfectionists.
The fact that you can cut a box volume in half and still produce a system that most people would find perfectly decent should make you understand that that you have a lot of leeway in building ported boxes. If you get things within a reasonable range, it will come out fine. Indeed, variations of 10% or less are almost never audible at all, let alone being bad enough to cause a rejection of the entire configuration.
I find that finding the "optimal" system is a good starting point, then I vary things within an acceptable range to my taste.
This is an example similar to one given by David Weems, who has been explaining how the Thiele-Small numbers work for decades through his book, "Designing, Building and Testing Loudspeakers".
Suppose you have a 10" driver with the following specs:
Fs = 30 Hz
Vas = 2 cu ft
Qts = .38, (or .4, for simplicity)
The "optimal" box will be a 2 cu ft box tuned to 30 Hz. The response will be flat with the F3, (-3 dB point), being 30 Hz.
Suppose we want to use this same driver in a smaller box. Suppose we want to use it in a 1 cu ft box.
Remember, a box half the size will give an F3 one half octave above any given box. So our F3 will be raised half an octave, from 30 Hz to 42 Hz. (To raise a frequency half an octave, you multiply it by 1.4-not 1.5. To raise it a full octave, you multiply it by 2).
Our box will then be:
Vb=1 cu ft
Fb, (frequency you tune the box) = something slightly below 42 Hz
F3 = 42 Hz.
However you will have a +2.3 dB hump in the octave above 42 Hz, and your transient response will be a little worse than the "optimal" box. But-it still can be a good, listenable box. +2.3 dB is not that bad. There is a formula for the correct box tuning, but if you make it 39 or 40 Hz-a little below your F3 of 42 Hz-you should be very, very close.
Another way of looking at that 1 cu ft box is to say that the best Qts for that box is about .32, and the hump is caused by having a Qts too high-.43 instead of .32. But again, the box will be listenable, except for perfectionists.
The fact that you can cut a box volume in half and still produce a system that most people would find perfectly decent should make you understand that that you have a lot of leeway in building ported boxes. If you get things within a reasonable range, it will come out fine. Indeed, variations of 10% or less are almost never audible at all, let alone being bad enough to cause a rejection of the entire configuration.
I find that finding the "optimal" system is a good starting point, then I vary things within an acceptable range to my taste.
Good Info there Keltic.
In addition, I noticed that if you make the box to match the VAS then the port velocity gets out of hand.
Simplified rule of thumb.
The smaller the box, the longer the port and the lower the port velocity, but poorer Transfer Function (FR) and harder to drive.
The larger the box, the shorter the port and the higher the velocity, but better Transfer Function and easier to drive.
Personally, I like the sound of a smaller enclosure (not sure why) say about ½ the VAS.
Truly, everything is a compromise
In addition, I noticed that if you make the box to match the VAS then the port velocity gets out of hand.
Simplified rule of thumb.
The smaller the box, the longer the port and the lower the port velocity, but poorer Transfer Function (FR) and harder to drive.
The larger the box, the shorter the port and the higher the velocity, but better Transfer Function and easier to drive.
Personally, I like the sound of a smaller enclosure (not sure why) say about ½ the VAS.
Truly, everything is a compromise
tech.knockout said:
Depends what you mean by "smaller". If the box size is smaller compared to it's Vas and it is tuned reasonably flat, it will give better transient response than a speaker put into a box equal to it's Vas. Of course, for a speaker to be put into a box much smaller than it's Vas and to be reasonably flat, it must have a Qts beneath O.4.
So a speaker with a Vas of 1 cubic ft, a Qts of .39 and an Fs of 40 Hz put into a 1 cu ft box and tuned to 40 Hz will give worse transient response than a speaker of Vas = 4 cu ft, a
Qts of 0.32 and an Fs of 28 Hz put into a 2 cu ft box and tuned to 35 Hz.
Even though both boxes have approximately the same F3, (-3 dB down point), the 2 cu ft box will give better transient response-because it is actually smaller compared to it's Vas.
In both cases, the transient response will be considerably worse than a closed box with a similar F3 cutoff and a similar flat frequency response.
Don't get too hung up on transient reponse. Remember that the lower you go in frequency, the less sensitive you are to distortion, "bass hangover" etc. It is so hard to get good bass output that if you must compromise, it is a good idea to compromise in the direction of output versus low distortion. Within reasonable limits, of course. Naturally, if you are going to build a small box with an F3 of 80, you would pay more attention to transient response than if you are building a large box with an F3 of 25. Most of the distotion occurs in the octave above the tuning frequency in a ported box or the resonance frequency in a closed box. So in a ported box tuned to 25 Hz, the distortion and "hangover" is basically confined to frequencies below 50 Hz-itself a pretty low frequency.
Will post graphs of "bass hangover" for various configurations of closed box and ported box in the next day or two so you can see what is happening. Remember, though, the ported box gives better output, and the lower you go, the less you can hear distortion. In the end, it comes down to how much extra output versus how much distortion you are willing to put up with.
In case your wondering, the driver im looking at now is the Peerless 850122.
I could have considered the Peerless HDS version of it, but it is much better suited to bookshelf/MTM designs, it doesnt work that well in a single bass driver floorstanding enclosure(Vas = 19L , large bass hump @ tuning freq if Vb>25L unless u tune really low).
Despite Kelticwizard saying in another thread that both drivers are similar, they seem to prefer very different enclosures and their Fs/Vas are very different.
edit: when tuned at the same freq and the same Vb, the results are 'very' similiar.
I guess Keltic might be right. the diff is the transient response. The CSX woofer is better here, dunno about how the HDS compares to the CSX in terms of 'definition'.
I could have considered the Peerless HDS version of it, but it is much better suited to bookshelf/MTM designs, it doesnt work that well in a single bass driver floorstanding enclosure(Vas = 19L , large bass hump @ tuning freq if Vb>25L unless u tune really low).
Despite Kelticwizard saying in another thread that both drivers are similar, they seem to prefer very different enclosures and their Fs/Vas are very different.
edit: when tuned at the same freq and the same Vb, the results are 'very' similiar.
I guess Keltic might be right. the diff is the transient response. The CSX woofer is better here, dunno about how the HDS compares to the CSX in terms of 'definition'.when both are tuned to the flat curve 'B4' alignment, the HDS can produce around 2Db more than the CSX above 40Hz before both drivers reach their Xmax.
Both can produce max 100Db at 40Hz. Thats the limit for both because both only have 5.5mm X-max and 6.5" diameter.
The CSX can produce the bass above 40Hz as loud as the HDS using all its X-max (when tuned differently). But its a 3Db bass hump that ends at 90Hz. Its midrange is 3Db quieter.
If u make it a flat curve, and then add more power so that it can produce the same amount of bass as the 3Db hump and be flat all the way to the midrange, it will exceed its Xmax.
The HDS has a flat curve (it is even from the midrange all the way to 50Hz). The CSX needs equalization to make use of its Xmax between 40-90Hz AND have a flat curve all the way to its midrange. thats why they charge us more for the HDS I think. lol I dont think many guys would get wot I said. Sorry i cant post the graphs. I use unibox 3.0
Both can produce max 100Db at 40Hz. Thats the limit for both because both only have 5.5mm X-max and 6.5" diameter.
The CSX can produce the bass above 40Hz as loud as the HDS using all its X-max (when tuned differently). But its a 3Db bass hump that ends at 90Hz. Its midrange is 3Db quieter.
If u make it a flat curve, and then add more power so that it can produce the same amount of bass as the 3Db hump and be flat all the way to the midrange, it will exceed its Xmax.
The HDS has a flat curve (it is even from the midrange all the way to 50Hz). The CSX needs equalization to make use of its Xmax between 40-90Hz AND have a flat curve all the way to its midrange. thats why they charge us more for the HDS I think. lol I dont think many guys would get wot I said. Sorry i cant post the graphs. I use unibox 3.0
There have been several discussions on this board about Peerless and it's relationship to published specs. The general consensus is that they can vary widely from published specs, but when placed into a box built for the published specs, they will give F3, etc decently close to what the published specs say.
This is for ported and closed boxes. I never built a Transmission Line, so I cannot say if things work out so well there.
Bob Brines is a member here, and he has a page where he documents his experience with Peerless 6.5's. How much variation there was for the pair he sent back, and whether he broke the first pair in or not, I cannot say.
http://geocities.com/rbrines1/
Madisound used to test batches of speakers as they came in-I don't know if they still do, (this was 10 years ago). A Peerless CSC subwoofer was listed with a Qts of .36, Fs of 30 and a Vas of about 2 cu ft.
Madisound measured a batch, and they averaged a much lower Qts and Fs and a much higher Vas. When placed into a 2 cu ft box, however, guess what-they gave essentially the same performance as the published specs. Only difference was that the tuning should be moved slightly lower.
Audax users have reported that their drivers give different specs from the published ones, too. I do not know if these differences compensate for each other when placed into a box the way Peerless does, however. Audax, like Peerless, has a reputation for giving you good sound for not a lot of money, in many models.
Two final things. I came across a webpage where somebody used the Peerless HDS and decided, subjectively, that it sounded "dead" musically. He also reported that he heard the same from others. This is just a webpage from a guy who built a speaker, take it for what it is worth. The guy seemed to know what he was doing with his speakers, though.
Also, I came upon a dealer website where the specs for the HDS were much closer to the CSX than the published specs. Whether the dealer measured these himself, a la Madisound, or whether the published specs just changed, I don't know. But there you go.
For these reasons, I generally regard the difference between the Peerless HDS line and the CSX line as being that the HDS line has a cast basket and a somewhat higher price, but otherwise similar.
This is for ported and closed boxes. I never built a Transmission Line, so I cannot say if things work out so well there.
Bob Brines is a member here, and he has a page where he documents his experience with Peerless 6.5's. How much variation there was for the pair he sent back, and whether he broke the first pair in or not, I cannot say.
http://geocities.com/rbrines1/
Madisound used to test batches of speakers as they came in-I don't know if they still do, (this was 10 years ago). A Peerless CSC subwoofer was listed with a Qts of .36, Fs of 30 and a Vas of about 2 cu ft.
Madisound measured a batch, and they averaged a much lower Qts and Fs and a much higher Vas. When placed into a 2 cu ft box, however, guess what-they gave essentially the same performance as the published specs. Only difference was that the tuning should be moved slightly lower.
Audax users have reported that their drivers give different specs from the published ones, too. I do not know if these differences compensate for each other when placed into a box the way Peerless does, however. Audax, like Peerless, has a reputation for giving you good sound for not a lot of money, in many models.
Two final things. I came across a webpage where somebody used the Peerless HDS and decided, subjectively, that it sounded "dead" musically. He also reported that he heard the same from others. This is just a webpage from a guy who built a speaker, take it for what it is worth. The guy seemed to know what he was doing with his speakers, though.
Also, I came upon a dealer website where the specs for the HDS were much closer to the CSX than the published specs. Whether the dealer measured these himself, a la Madisound, or whether the published specs just changed, I don't know. But there you go.
For these reasons, I generally regard the difference between the Peerless HDS line and the CSX line as being that the HDS line has a cast basket and a somewhat higher price, but otherwise similar.
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